Binders for the electrostatic powder-spraying process

ABSTRACT

COPOLYMERS OF 25 TO 60% BY WEIGHT OF STYRENE OR ITS DERIVATIVES, 30 TO 65% OF (METH) ACRYLIC ESTERS (EXCEPTED (METHACRYLIC ACID MTHYL ESTER) WHICH MAY BE USED IN AMOUNTS UP TO 24% BY WEIGHT) AND 3 TO 15% BY WEIGHT OF UNSATURATED COPOLYMERIZABLE CARBOXYLIC ACIDS ARE HOMEGENEOUSLY MIXED WITH TRIGLYCIDYL ISOCYANURATE AND OPTIONALLY WITH PIGMENTS AND LEVELLING AGENTS. THE MIXTURE IS APPLIED BY ELECTTROSTATIC POWDER-SPRAYING PROCESS TO METALLIC SURFACES AND CURED AT TEMPERATURES OF ABOUT 130 TO 220* C. THE BACKED FILMS OF THE MIXTURES SHOW OUTSTANDIING BOND STRENGTH AND HARDNESS COUPLED WITH ELASTICITY, HIGH GLOSS AND WEATHER RESISTNACE.

United States Patent US. Cl. 260-836 1 Claim ABSTRACT OF THE DISCLOSURE Copolymers of 25 to 60% by weight of styrene or its derivatives, 30 to 65% of (meth)acrylic esters [excepted (methacrylic acid methyl ester) which may be used in amounts up to 24% by weight] and 3 to 15% by weight of unsaturated copolymerizable carboxylic acids are homogeneously mixed with triglycidyl isocyanurate and optionally with pigments and levelling agents. The mixture is applied by electrostatic powderspraying processes to metallic surfaces and cured at temperatures of about 130 to 220 C. The backed films of the mixture show outstanding bond strength and hardness coupled with elasticity, high gloss and weather resistance.

It is known from German Auslegeschrift No. 1,180,373 that triglycidyl isocyanurate can be reacted with polycarboxylic acidsto form crosslinked products.

It is also known from British Patent Specification No. 773,206 that solution polymers of styrene, acrylic acid and butyl acrylate can be used in conjunction with triglycidyl cyanurate as binders in solvent-containing lacquers. Binders for a solvent-containing lacquer system cannot readily be used for the electrostatic powder-spraying process because special properties are required of the binder in this process.

In their uncrosslinked state, binders for the electrostatic powder-spraying process should be brittle, readily pulverisable resins which, as powders, remain free-flowing and lump-free at temperatures of up to 60 C. After electrostatic application to the substrate, the powders should spread evenly at temperature of from 80 to 120 C. and should be suitable for baking at temperatures upwards of 130 C. to form insoluble and infusible coatings. In addition, the powders should be able to accept an electrostatic charge and to hold the electrical charge on metal articles for a while before baking. In addition, binders of this type should not crosslink prematurely in the vicinity of their levelling temperature because they are generally mixed in the melt with pigments, catalysts and levelling agents at temperatures around 100 C. Where an external crosslinker is used, as in the process according to the invention, the component acting as crosslinker should melt at temperatures at which the resin itself melts and should be compatible and homogeneously miscible with the resin. When the mixture solidifies, the crosslinker added should not adversely affect the stability of the mixture in storage at temperatures up to 60 C., nor should the system disintegrate again on cooling. After levelling and baking, the compatible mixture of crosslinker and resin should give a high-gloss weather-resistant coating which is unaffected by chemicals and solvents.

Powder lacquers based on polyepoxides are known and largely satisfy the requirements outlined above, although they lack suflicient weather resistance for many applications. The inadequate weather resistance of epoxide-based powder lacquers prompted the development of acrylatebased powder lacquers. Acrylate powder resins of this kind are described for example in French Patent Specification No. 2,035,185. Resins of this type contain in the molecule firmly anchored methylmethylol ether groups which permit thermal crosslinking. Unfortunately, resins such as these show inadequate stability to storage at elevated temperatures of the kind which can occur in particular during transportation in warm countries. The resins generally begin to lump at temperatures as low as 40 C. and, as a result, are no longer free-flowing. However, permanent free-flow properties are essential to any process of powder application.

The present invention relates to the use of binders of mixtures of copolymers of styrene, (meth)acrylates and an tarp-unsaturated carboxylic acid with triglycidyl isocyanurate for the electrostatic powder-spraying process, in which the coating powders which preferably have a grain size of from about 30 to 120, and remain freefiowing at temperatures of at least 50 C. consist of (A) polymerized units of I. 25 to 60% by weight of styrene, a-methyl styrene,

o-chlorostyrene, p-chlorostyrene, p-tert.-butyl styrene or mixtures thereof;

II. 30 to 65 by weight of acrylic esters containing 1 to 12 carbon atoms in the alcohol radical or methacrylic esters containing 2 to 12 carbon atoms in the alcohol radical or mixtures thereof;

III. 0 to 25% by weight of methacrylic acid methyl esters;

IV. 3 to 15% by weight of acrylic acid, methacrylic acid, itaconic acid, maleic acid or fumaric acid,

the total sum of the percentage contents I to IV amounting to whilst the average molecular weight is between about 3000 and 20,000,

(B) triglycidyl isocyanurate and may also contain (C) additives such as pigments and levelling agents, and

further by the fact that for 95 to 80 parts by weight of acrylic resin (A) there are 5 to 20 parts by weight of triglycidyl isocyanurate (B), and may also contain up to by weight, based on (A) and (B), of pigments and from 0.1 to 5% by weight, based on (A), of a levelling agent.

In the context of the invention, the expression polymer powders covers not only copolymers with the composition defined above, but also mixtures of copolymers providing these mixtures are prepared in such a way that the overall composition lies within the aforementioned ranges.

The copolymers are produced by conventional bulk polymerisation, solution polymerisation, dispersion polymerisation and bead polymerisation processes, preferably by solution or bulk polymerisation. Processes of this kind are described for example in Methoden der Organischen Chemie, I-Iouben-Weyl, 4th Edition, Vol. 14/ 1, pages 24?- 556 (1961).

In cases where polymerisation is carried out in solution, such solvents as methylene chloride, ethanol, isopropanol, n-propanol, n-butanol, isobutanol, tert.-butanol, acetic acid methyl up to butyl ester, acetone, methyl ethyl ketone, benzene, toluene, etc. can be used.

The polymerisation reactions are carried out at temperatures of from 40 to about C.

Examples of suitable initiators include percarbonates, peresters such as tert.-butyl perpivalet and peroctoate, benzoyl peroxide, o-methoxy benzoyl peroxide, dichlorobenzoyl peroxide, azodiisobutyrodinitrile, being used in quantities of from 0.5 to 3% by weight, based on monomer.

The usual molecular weight regulators such as thioglycol, thioglycerin or tert.-dodecyl mercaptan, can also be used.

The solvent is removed from the copolymer solution in a suitable apparatus, preferably in evaporator screws, at

temperatures of from about 90 to 180 C., followed by cooling, granulation and grinding, for example by the process disclosed in Belgian Patent Specification No. 762.,- 586. However the copolymers can also be isolated by other methods, for example by spray drying, removal of solvent with steam accompanied by dispersion in Water or by precipitation with water from a solvent miscible with water.

After they have been isolated, the copolymers can be mixed with up to 150% by weight and preferably with up to 100% by weight of inorganic or organic pigments, based on the copolymer, at temperatures of fro-m 80 to 120 C. Examples of suitable pigments include titanium dioxide, iron oxides, chromium oxide, phthalocyanine and azo pigments.

Levelling agents such as silicones, polyesters, cellulose derivatives and also catalysts such as organic and inorganic acids, tertiary amines, dicyanodiamine and tin compounds can additionally be added to the melts (0.1 to 5% by weight, based on copolymer).

It is of course also possible to add pigments and levelling agents to the monomers before polymerisation or, in the case of solution polymerisation, to the copolymer solution before the solvent is removed from it.

The triglycidyl isocyanurate is preferably incorporated into the mixture in a mixing screw at temperatures of from 80 to 120 C. together with the pigments, levelling agents and catalysts. The mixing time should amount to between about 1 and minutes. Prolonged mixing results in pre-crosslinking of the product. Under these conditions a homogeneous mixture which does not disintegrate, even when cooled, is obtained.

The soivent-free mixtures which may contain pigments and which in their uncrosslinked state, are brittle after cooling are ground to a grain size of from about 30 to 120 and may be separated according to grain size.

The polymer powders used in accordance with the invention remain free-flowing at temperatures of up to at least 50 C., preferably 60 C., have levelling temperatures of from about 80 to 120 C. and are baked for to minutes at temperatures from about 130 C. to 220 0, preferably at temperatures of from 160 to 180 C., which baking results in cross-linking.

'The polymer powders have average molecular weights of from about 3000 to 20,000 as measured in an organic solvent by the vapour-pressure reduction method.

The powders are applied to suitable substrates, especially metals, by conventional electrostatic powderspraying processes (40 to 90 kv.), cf. D. R. Davis Coating With Electrostatic Drey-Spray in plastics technology, June 1962, pages 37-38.

The baked films (40 to 300 thick) of the polymer powders used in accordance with the invention show outstanding bond strength and hardness coupled with elasticity. They are also distinguished by their high gloss and weather resistance.

The powders are used for coating domestic appliances, metal components in automobile construction and metal components exposed to heavy weathering, such as automobile bodywork, facade panels, tubes, wire netting and machines of the kind used in forestry and agriculture.

The production of the powders and their use as electrostatically sprayable powders are described in the following Examples in which the parts and percentages quoted relate to weight unless otherwise stated.

Production of the polymer powder 8 kg. of a monomer mixture consisting of 50 parts by Weight of styrene, parts by weight of butyl acrylate, 15 parts by weight of acrylic acid, 1 part by weight of tert.-dodecyl mercaptan and 0.005 part by weight of hydroquinone, are heated under nitrogen in a litre vessel equipped with stirring mechanism until gentle refluxing "begins. A temperature of from 130 to 140 C. is

reached. A cooled mixture of 15 g. of tert.-butyl perpivalet in a petroleum fraction (bp 70l30 C.) and 640 g. of the monomer mixture is introduced over a period of 1 hour into the initial monomer mixture, accompanied by polymerisation. The solids content now amounts to (as determined by concentrating a sample by evapo ration for 30 minutes at 250 C.). If the reaction becomes too vigorous, stopping the introduction of the initiator is sufficient to terminate polymerisation within a few seconds. A mixture of 16 kg. of monomer mixture and 160 g. ,of tert.-butyl perpivalet (the components are mixed together shortly before introduction in a vessel cooled with brine) is introduced into the melt over a period of 3 hours at to C., the reaction mixture heated to C. on completion of the reaction and 180 g. of di-tert.-butyl peroxide and 180 g. of monomer mixture added dropwise over a period of another hour. The volatile constituents (catalyst decomposition products, monomer impurities) are then distilled off and the temperature subsequently kept at C. for 2 hours. Thereafter, the melt is briefly degassed in vacuo and run ed. The lacquer has a melting point of 95 C. and can be pulverised without lumping. It has an average molecular weight of approximately 6000, as determined by vapour-pressure reduction method (osmometry).

EXAMPLE 1 A resin produced by the process described above, consisting of copolymerised units of 50% by Weight of styrene, 15% by weight of acrylic acid and 35% by weight of butyl acrylate, with a molecular weight of 6000 and a residual monomer content of less than 0.2% by weight, is ground to a grain size of about 300 and mixed with 10 parts of weight of triglycidyl isocyanurate, based on 100 parts by weight of resin, and with 100 parts by weight, based on resin-hardener mixture, of titanium dioxide (rutile). The mixture is homogeneously mixed for 2 minutes in a pigmenting screw at a barrel temperature of 100 C. and an outlet temperature of 120 C., and the melt cooled by spreading it over cooling plates.

The melt cake is ground to a grain size of approximately 60 .4 in a breaker-plate mill. Even after storage for 48 hours at 55 C., the pulverised mixture is freefiowing and only lumps after heating to temperatures beyond 60 C.

The powder is electrostatically sprayed on to degreased iron plates in a conventional spraying machine under a voltage of 40 kv. (delivery pressure 0.8 atms). The coated plates are then heated for 30 minutes to C. The films have a thickness of approximately 80 1.. The films on the test plates have a Gardner gloss of 90 C., an Erichsen indentation of 8 mm. (DIN 53,156) and are unmarked after contact with a toluene-impregnated cotton wool pad followed by regeneration for 2 minutes.

EXAMPLE 2 The procedure is as in Example 1 except that a resin of the following composition is used: 25% by weight of styrene, 10% by weight of butylmethacrylate, 15% by weight of methylmethacrylate, 15% by weight of acrylic acid, 35% by weight of butyl acrylate. The polymer is prepared by the polymerisation process described in Example l.

The following data were measured on plates coated electrostatically as in Example 1:

Erichsen indentation 7 mm.

Gardner .gloss 95 C.

Solvent resistance Unmarked by toluene after regeneration for 2 minutes.

Weather resistance 700 hours in the Sunshine Weather-Ometer test. Stability of the pigmented powderinstorage 48 hours at 55 C., no

lumps.

EXAMPLE 3 95 parts by weight of a resin produced by the process as described above and consisting of copolymerised units of 57.5% by weight of styrene, 7.5% by weight of acrylic acid and 35% by weight of butylacrylate having an average molecular weight of 5500, measured by the vapour pressure osmometric method are mixed with 5 parts of triglycidyl isocyanurate, 0.2 parts by weight of stannous dioctoate and 20 parts by weight of carbon black, ground to a grain size of about 300,u and homogeneously mixed for 45 seconds in a pigmenting screw at 105 C., cooled and ground to a grain size of about 60 The powder is electrostatically sprayed as described above on a bicycle frame and backed for 15 minutes to 200 C. The film shows an outstanding high gloss and good resistance to petrol and weathering.

We claim:

1. A powdered coating agent comprising a mixture of:

(A) a copolymer of (I) 25-60% by weight styrene, u-methylstyrene,

o-chlorostyrene, p-chlorostyrene, p-tert.-butylstyrene or mixtures thereof;

(11) 30-65% by weight of acrylic acid esters having 1-12 carbon atoms in the alcohol residue or methacrylic esters having 2-12 carbon atoms in the alcohol residue or mixtures thereof;

(III) 025% by weight methylmethacrylate; and

(IV) 315% by weight of acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid or mixtures thereof;

wherein the sum of the percentage contents of (I) to (IV) is and the average molecular weight of said copolymer is 3000 to 20,000,

(B) triglycidyl isocyanurate and optionally (C) auxiliary agents such as pigments and levelling agents wherein there are 95 to 80 parts by weight of said copolymer A to 520 parts by weight of said triglycidyl isocyanurate (B) as well as optionally not more than by weight, based on (A) and (B) together, of pigments and 0.1 to 5% by weight, based on (A), of a levelling agent.

References Cited UNITED STATES PATENTS 3,652,723 5/1972 Fellers 260--837 2,604,464 7/1952 Segall 260837 3,215,756 11/1965 Lombardi 260-836 3,305,601 2/1967 Hicks 260-836 3,400,098 9/1968 Parry 260837 FOREIGN PATENTS 773,206 4/1957 Great Britain 260-836 935,720 9/1963 Great Britain 260-837 941,732 11/1963 Great Britain 260836 PAUL LIEBERMAN, Primary Examiner US. Cl. X.R.

1l7l32 BE; 26037 Ep, 41 B, 41 C, 824 Ep, 827, 835 

